In association with explosive increase in demand for broadband multimedia communication services such as the internet and video delivery, introduction of long-haul, large-capacity and high-reliability fiber optic communication systems has been advanced. In fiber optic communication systems, it is important to reduce the construction cost of optical fibers to become the transmission lines, and to increase the efficiency of transmission band utilization per one optical fiber. Accordingly, the importance of digital coherent optical communication technology using a digital optical transmitter/receiver has been increasing.
In digital coherent optical communication, waveform distortion, such as wavelength dispersion, is compensated by performing digital signal processing (DSP) at a sending side or a receiving side. As a result, digital coherent optical communication can perform such compensation with higher accuracy, compared to analog optical transmitters/receivers using modulation methods such as OOK (on-off keying), which are generally employed in large-capacity optical communication systems. Therefore, digital coherent optical communication enables to realize performance improvement and cost reduction of communication devices. Examples of optical communication devices employed in digital coherent light communication are disclosed in Patent Literature 1 and Patent Literature 2.
FIG. 19 shows a block configuration diagram of a general optical transmitter employed in digital coherent optical communication. Being different from binary data modulation performed in analog transmitters, modulation performed in the digital optical transmitter of FIG. 19 is such as multi-level modulation using QPSK (quadrature phase shift keying), QAM (quadrature amplitude modulation) or the like, or arbitrary waveform modulation using output of a D/A (digital to analog) converter in order to perform pre-equalization or the like.
In such a digital optical transmitter, a Mach-Zehnder (hereinafter, referred to as MZ) type optical modulator is generally used. An MZ type optical modulator is formed by installing optical-waveguide type optical phase modulators into an optical-waveguide type MZ type interferometer. In such an MZ type optical modulator, various kinds of optical modulation including intensity modulation and phase modulation are performed by adjusting the applied voltage and the interferometer configuration.